The present invention relates generally to gas supply units and methods for supplying inert gas to an exposure apparatus, and exposure systems using the same. In particular, the present invention is suitable for a gas supply unit, as well as to an exposure system, for supplying inert gas to an exposure light path of a projection exposure apparatus that uses far UV light and an excimer laser beam as a light source.
Along with recent demands on smaller and lower profile electronic devices, fine semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. The conventional printing or photolithography for fabricating semiconductor devices has used a projection exposure apparatus.
In general, a projection exposure apparatus includes an illumination optical system that uses light emitted from a light source to illuminate a mask, and a projection optical system arranged between the mask and an object to be exposed. For a uniform illumination area, the illumination optical system introduces light from a light source into a light integrator, such as a fly-eye lens composed of multiple rod lenses, and uses a light exit plane of the light integrator as a secondary light source plane to Koehler-illuminate the mask plane through a condenser lens.
The minimum critical dimension to be transferred by the projection exposure apparatus (resolution) is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture of the projection optical system. The shorter the wavelength is, the better the resolution is.
Accordingly, the light source in recent years has been in transition from an ultra-high pressure mercury lamp (g-line with a wavelength of approximately 436 nm) and i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm). Practical use of F2 excimer laser (with a wavelength of 157 nm) has been promoted.
It is known that i-line or other exposure light with a shorter wavelength results in a photochemical reaction between the impurity in the air and oxygen (O2) due to its short wavelength, which generates products to adhere to and opaque an optical element, such as a lens and a mirror in an optical system.
The products typically include ammonium sulfate ((NH4)2SO4), for example, which is produced by sulfuric acid (SO2) that reacts with oxygen in the air or oxidizes when it absorbs light energy and gets excited. Ammonium sulfate is whitish and opaques an optical element, such as a lens and mirror, when it adheres to a surface of the optical element. Ammonium sulfate disperses and absorbs the exposure light, and lowers the transmittance of an optical system, thus greatly reducing an exposure light intensity or transmittance down to an object to be exposed and throughput.
The far UV light, such as excimer laser with a wavelength of 250 nm or less, particularly, ArF excimer laser having an oscillation wavelength of about 193 nm includes multiple oxygen absorption bands in this wavelength region. For example, as shown in FIG. 10, inert gas supplied from a plant facility 1100 is supplied to a tube port 1210 in an exposure apparatus 1200 to purge its optical system and reduce oxygen concentration in the exposure light path to a very low level for exposure light with a less absorbent and purified oscillation wavelength. Here, FIG. 10 is a schematic block diagram of a conventional exposure apparatus.
It is also known that the F2 excimer laser with an oscillation wavelength of about 157 nm includes consecutive oxygen absorption bands in this wavelength region, and does not allow exposure light with a less absorbent wavelength to be selected like the ArF excimer laser. The vacuum UV light with a wavelength of about 157 nm includes continuous steam absorption bands that cannot be observed around 193 nm. The vacuum UV light with 157 nm is easily absorbed by ammonia (NH3), carbon dioxide (CO2), organic gases, etc., and a light absorption in the exposure light path increases substantially, which is not a problem for the vacuum UV light with a wavelength of 160 nm or less.
A fluctuant concentration of a light absorbent material in the exposure light path during exposure would result in an error or discord of the actual exposure dose relative to the target exposure dose, and deteriorate the above throughput and an exposure-dose control precision.
Accordingly, the impurity concentration should be monitored in gas constituents in the exposure light path in a projection exposure apparatus that uses the far UV light or excimer laser for controls over optical systems in their product adhesion, efficiency and the exposure dose.
However, the conventional exposure apparatus shown in FIG. 10 cannot detect the impurity concentration of the supplied gas, and might cause the projection exposure apparatus to accept the inert gas, etc. with an impermissible impurity concentration due to malfunctions etc. of the plant facility. The inert gas, etc., with an impermissible impurity concentration supplied to the exposure apparatus would cause the following disadvantages:
(1) The light absorption increases in the exposure light path and considerably lowers the throughput of the apparatus. (2) The fluctuant light absorption in the exposure light path during an exposure operation causes a change or error in the actual exposure dose to the target exposure dose, and deteriorates the exposure-dose control accuracy. (3) Impurities in the inert gas, etc. in the exposure light path photochemically react and cause resultant products to adhere to an optical element, such as a lens and a mirror in an optical system. The products lower performance, such as the optical efficiency, and might require an exchange for an expensive optical element depending on adhesions. (4) The impurities adhere to a pipeline system for guiding the inert gas, etc. to the exposure light path, and might require its cleansing or exchange.